An emergency or normal shutdown of any high-temperature nuclear reactor creates a need for a system to remove excessive decay heat. Nuclear reactors produce, during the course of their normal operation, radioactive materials which decay and produce heat for a period of time after the reactor is shutdown. Sufficient coolant must continue to circulate for a lengthy period of time to remove that heat to prevent damage to the reactor and associated systems. A power generating nuclear reactor, for instance, is generally provided with a steam generator which acts as a heat sink during normal operation. Therefore, a means must be present to provide an alternate heat sink when the steam generator is not available.
U.S. Pat. No. 4,699,754 describes one system for removing decay heat from a reactor core which has a liquid metal coolant circulation system. Typical coolants for these type of reactors are sodium or potassium which, during operation, may reach temperatures in the order of 1200.degree. to 1500.degree. K. Previous reactors of this type have used an auxiliary Thermoelectric Electromagnetic Pump in an auxiliary flow path connected in parallel to a portion of the primary flow path. These previous reactors had a check valve in the primary flow path between the connections for the auxiliary path. During normal operation, a primary cooling pump flow holds that check valve open. However, the auxiliary Thermoelectric Electromagnetic Pump maintains a coolant flow through the auxiliary flow path with the check valve preventing back-flow if the primary cooling pump stops. U.S. Pat. No. 4,699,754 mentions that moving parts such as check valves are unreliable when subjected to high-temperatures and held in one position for long periods. In order to avoid the necessity for this check valve, U.S. Pat. No. 4,699,754 suggests using the Thermoelectric Electromagnetic Pump in the auxiliary flow path to re-inject a secondary stream of metal coolant into the main coolant stream. The re-injection acts as a drive fluid for a jet pump in the main flow path which, using the principal of momentum exchange, induces a circulation of the main fluid in the same direction as the normal primary coolant flow. This provides an auxiliary circulation system without any moving parts and which is self-regulating. The auxiliary Thermoelectric Electromagnetic Pump/jet pump combination operates during normal operation of the reactor but the flow in the auxiliary flow path is small compared to the main flow so that the systems efficiency is not greatly diminished.
U.S. Pat. No. 4,689,194 shows another type of decay heat removal system which, in this case, is for a gas cooled reactor. Circulating blowers cause a cooling gas, such as helium, in this reactor to flow up through the reactor core and a central hot gas line down over principal heat exchangers, these may be steam generators, and decay heat exchangers back to the blowers. If the circulating blowers are not operational, decay heat from the core is removed by natural convection flow of the cooling gas in the same direction as the flow during normal operation of the reactor. The decay heat exchangers are each connected with an external re-cooling heat exchanger at a geodetically high location by means of two legs which form a water circulation loop. If the steam generators are no longer available for the removal of heat from the primary (helium) cooling path, they are traversed by hot gas which subsequently passes through the decay heat exchangers. This causes a rise in temperature at the inlet of the decay heat exchangers which leads to evaporation taking place in the water circulation loops whereby natural convection in these loops is enhanced and a sufficient amount of heat is removed from the primary loop through the decay heat exchangers.
U.S. Pat. No. 4,312,703 describes another type of system for removing heat from a nuclear reactor employing liquid sodium as a primary cooling fluid along with means for dissipating the decay heat produced in the core of the nuclear reactor after it has been switched off. In this system, a pump draws the liquid sodium coolant from the reactor vessel and transports it to an integrated intermediate heat exchanger and decay heat cooler before the coolant is returned to the reactor vessel. A secondary coolant fluid, also liquid sodium, in the intermediate heat exchanger receives heat from the primary cooling fluid with the secondary cooling fluid being pumped to a steam generator and back to the intermediate heat exchanger during normal operation of the reactor. A separate or third cooling circuit is integrated into the intermediate heat exchanger and forms a decay heat cooler in which a third cooling fluid can flow to a cooler component (air cooler or steam generator), then to a pump which circulates the third cooling fluid back to the intermediate heat exchanger to remove decay heat generated when the reactor is shut down. This structure provides a very compact construction compared to previous systems wherein a decay heat cooler is incorporated as a separate heat exchanger in the primary circuit.